A high image quality color hard copy printing system using a dye thermal transfer system is composed of a printer, a thermal transfer sheet (hereinafter simply referred to as an ink ribbon) and a receiving sheet. As a general method for forming a color image on the receiving sheet, the ink ribbon having color material layers composed of three or four colors i.e. yellow, magenta, cyan and, if necessary, black, is closely contacted with the receiving sheet, which is provided with an image receiving layer (hereinafter, simply referred to as receiving layer), and they are transported through a heating device and pressed by constant pressure from a platen roller. Then, heat generating part of the heating device is selectively made to generate heat, and the dye contained in the color material layers of the ink ribbon is transported to the receiving layer of the receiving sheet to obtain an image. An ink ribbon comprises color material layers consisting of three or four different colored-layers stacked on each other. A color image is formed by sequentially transferring each color, three or four times for each, at the same position of the receiving sheet, thereby superposing each color on the same position. In such a printer of a dye thermal transfer system, the receiving sheet is usually supplied in a sheet state.
Generally, the receiving sheet is formed of a sheet-like support, with a receiving layer formed on one surface thereof, having an excellent dyeing property with dye transferred from an ink ribbon, and a back surface layer formed on the other surface, and provides various properties such as printability in an ink jet printer and writability with various writing instruments.
In the receiving sheet, writing is occasionally performed on the surface (back surface side) opposite the surface of the receiving layer on which an image is formed. It is necessary that writing can be performed by various writing instruments. Recently, printability by an ink jet printer is also required. When the support is a synthetic paper or a plastic film, the support is insufficient in printability in an ink jet printer and is also insufficient in writability for various writing instruments. In order for the back surface layer to have excellent writability, it is necessary that the back surface layer exhibits sufficient writability for oily- or water-based pen-type writing instruments, pencils or the like. Moreover, good printability is also required for printing by a printer, such as an ink jet printer.
As a back surface layer of the thermal transfer receiving paper, some proposals have been made for the purpose of providing writability for various writing materials. In order to improve writability, the back surface layer containing an inorganic pigment includes, for example, a back surface layer containing plate-like barium sulfate (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-24676 (page 2)), a back surface layer containing silica microparticles having a specific particle size (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 3-180393 (page 1)), and a back surface layer containing microsilica and acetyl cellulose (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 8-244362 (page 2)).
However, in the back surface layer containing these inorganic pigments, since the inorganic pigments are generally hard, there is a problem that the ink receiving layer may be damaged because the surface of the ink receiving layer is rubbed with the back surface layer when the ink receiving sheet travels through a thermal transfer printer, resulting in poor printing quality. Moreover, printability and drying in an ink jet printer are also insufficient and an improvement is required.
In order to solve the above problem, a back surface layer with low hardness, containing an organic pigment, has also been proposed. For example, there is a back surface layer containing spherical silicone microparticles (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 7-108775 (page 2)), a back surface layer containing cellulose microparticles (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-92669 (page 2)), a back surface layer containing organic and/or inorganic particles, a higher aliphatic salt, and a binder as main components (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 1-241491 (Page 1)), and a back surface layer containing particles of a Mohs hardness of 1 to 4 (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 6-239036 (Page 2)). However, in the back surface layer containing an organic pigment, because of slight damage to the receiving layer by the back surface layer, pencil writability is insufficient, and also printability and drying in an ink jet printer are also insufficient.
A proposal to improve writability by mixing inorganic or organic pigment with the back surface layer improves writability by conventional writing instruments such as water- and oil-based pens and pencils to a certain degree. However, in printing with an ink jet printer, sufficient printability and drying cannot be obtained, and performance has been unsatisfactory. In printing with an ink jet printer, because a large amount of water is provided on the back surface layer, it is necessary to design the back surface layer taking into consideration water absorption, drying, and coating film strength, as well as printability.
From these viewpoints, there has been proposed a back surface layer having writability by conventional writing instruments and printability in an ink jet printer (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2001-199172 (page 2), Japanese Unexamined Patent Publication (Kokai) No. 2001-213057 (Page 2), and Japanese Unexamined Patent Publication (Kokai) No. 2003-191652 (page 2)). However, in these back surface layers, general recording is possible by an ink jet printer, but clear image quality cannot necessarily be obtained. Moreover, because the stability of the back surface layer coating material is insufficient and the cost is high, more improvement is required.
Moreover, as the binder for the back surface, a water-soluble resin, such as polyvinyl pyrrolidone is exemplified (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2000-185476 (pages 7-8)) or Japanese Unexamined Patent Publication (Kokai) No. 2002-337463 (Page 4). However, when a single-member back surface layer is formed by using a water-soluble resin as a main binder, if printing is performed with an ink jet printer, the drying of the ink is insufficient, and therefore, improvement is required.